The present invention relates to a tapping device for forming a female thread in a workpiece, and more particularly, to a control device for controlling rotation of a feed motor and a spindle motor of a tapping device for controlling rotation speed and axial moving speed of a spindle to which a composite tool having a tip boring section and a subsequent tapping section is attached.
In a conventional tapping device, a spindle holds a composite tool having a boring section and a tapping section. The boring section is positioned at a tip end portion of the tool for forming a bore in the workpiece, and the bored portion is then subjected to tapping by the subsequent tapping section. Thus, boring and tapping can be performed continuously without changing tools.
Laid open No. Hei-10-15733 of Japanese patent application (Kokai) discloses a synchronous control in which rotation number of the spindle and feed speed thereof in a Z-axis direction are synchronized with each other, and the feed speed and the rotation numbers in the boring are switched to different feed speed and rotation numbers in the tapping.
According to the synchronous control, spindle rotation number is obtained by dividing the Z-axis feed speed by the thread formation pitch. Further, the thread formation pitch in the boring is altered to a different thread formation pitch in the tapping by a function generator in order to provide the speed and rotation numbers those required in the boring and tapping, respectively.
According to the conventional device, since tool changing work is not required for boring and tapping, entire tapping work can be performed within a shorter period in comparison with a case where a boring tool and a separate tapping tool are required. Thus, machining efficiency can be improved. Further, different Z-axis feed speeds and rotation numbers are provided for the boring and the tapping, and therefore, optimum feed speeds and rotation numbers can be provided in the boring and tapping operation, which facilitates entire machining.
However, in the synchronous control, spindle rotation control is performed based on the Z-axis feed control. Therefore, a time delay may be provided in the spindle rotation control when responding to the Z-axis feed command after the Z-axis feed control. As a result, a swell may occur in the rotation numbers of the spindle due to the time delay, to degrade a machining accuracy. More specifically, such a time delay may not occur if rotation number and Z-axis feed speed of the spindle are maintained unchanged from the start to the end of the synchronous control. However, as described above, thread formation pitch is changed by the function generator to provide a first rotation speed at boring different from a second rotation speed at tapping. Therefore, at the pitch changing phase for changing the pitch for boring control to the pitch for tapping control, the spindle rotation number cannot be promptly converged to the second rotation speed but an overshoot may occur. Consequently, a swell of rotation number may occur, which degrade machining accuracy.